How to Make It
Preheat the oven to 400°. In a large skillet, cook half of the bacon over moderate heat, turning once, until crisp, about 6 minutes. Drain on paper towels. Repeat with the remaining bacon.
In a medium skillet, bring the water and olive oil to a simmer. Add the garlic in an even layer, cover and cook over moderate heat until the water has evaporated, about 5 minutes. Uncover and cook over moderate heat, undisturbed, until the garlic begins to turn golden, about 5 minutes longer. As the garlic colors, transfer it to a plate. Pour 1 tablespoon of the garlic oil into a medium bowl. Whisk in the mustard and vinegar and season with salt and pepper.
Arrange the bread on a baking sheet and top each slice with 4 slices of bacon. Cover the bacon with the cheese. Bake for 4 minutes, until the bacon is sizzling and the cheese is melted.
Add the watercress and the garlic slices to the dressing in the bowl and toss well. Mound the watercress on 4 of the cheese melts and close the sandwiches; cut in half and serve.